1. Field of the Invention
This application relates broadly to cold-seal adhesives, cold-sealable films, methods of forming packages therewith and packages sealed with the new cold-seal adhesives. More particularly, it concerns (a) polyurethane containing aqueous dispersion adhesives capable of forming dry, non-blocking adhesive layers on plastic film or equivalent webs that can then later be pressure bonded together without use of heat, (b) cold-sealable packaging films comprising such adhesive layers and (c) packages formed and sealed with such packaging films.
2. Description of the Prior Art
Cold-seal adhesives are an established class of commercial adhesives characterized by being coated onto plastic films or other substrata to form layers that have the capability of strongly bonding to themselves when pressure is applied, but lacking adhesion to low energy surfaces, e.g., plastic films, coated papers, etc., so such flexible webs bearing these cold-seal adhesives layers may be formed into rolls without blocking, i.e., without sticking together. The technical literature contains information about cold-seal adhesives which have become a recognized commercial commodity, e.g., see U.S. Pat. Nos. 4,810,745, 4,859,521, and 4,902,370.
Natural rubber remains a major component of choice for the production of cold-seal adhesives in spite of disadvantages associated with natural rubber latex, including age discoloration, unpleasant odor, undesirable foaming in wet form and possibility of anaphylactic shock due to presence of natural latex proteins. To overcome the problems associated with natural rubber, synthetic polymer dispersions have been used to replace it in cold-seal adhesive formulations as shown by U.S. Pat. Nos. 4,889,884 & 4,902,370. While limited success has been accomplished in mitigating the aforementioned problems, the fundamental balance of satisfactory cohesive bond strength vs antiblocking properties desired in cold-seal adhesives has proven hard to achieve on a commercial basis without use of natural rubber.
The present invention provides new forms of cold seal adhesives with remarkable balance of satisfactory cohesive bond strength vs antiblocking properties without use of natural rubber as an essential component.
Aqueous polyurethane dispersions are a known class of polymer systems as are various methods for their production and their use as coatings and adhesives, as shown by U.S. Pat. Nos. 4,540,633, 4,623,416, 4,851,459, 4,861,826 & 5,334,690.
The success of the present invention is due in part to the discovery that the inclusion of colloidal silica chemically grafted onto a functional polyurethane prepolymer in the new of cold-seal adhesives of the invention critically improves the non-blocking qualities. The inclusion of such --NCO grafted silica in the new cold-seal adhesive is to be contrasted to the use of colloidal silica per se, as well as hydrophilic and hydrophobic versions thereof, as additives in adhesive composition as shown by U.S. Pat. Nos. 3,993,847, 4,710,536, 4,749,590, 5,348,923 and 5,459,185.
The present invention builds on the prior knowledge discussed above to advance the art in production of cold-seal adhesives and their utilization in cold-sealable packaging films plus packages made therefrom and sealed without recourse to heat sealing. Essentially, it is an improvement of a related invention disclosed in U.S. Pat. No. 5,616,400, assigned to the assignee of this patent application.